We were communicating via e-mail so for others:
In this case we have fixed the issue by setting:CMSSiteManager -> Settings -> URLs and SEO -> Files friendly URL extension: .aspx
The upgrade procedure configured this new property to extension-less URLs for files. It did not work on the site because the IIS was configured to handle only standard ASPX extensions.
Other fix is to configure IIS to handle extension-less URLs. Please see the documentation: Configuration of extension-less URLs